Mobile communication that has originated from the discovery of radio waves has been under research and development because of the necessity of communication with vessels, aircrafts, and trains. The targets of communication have been expanded into automobiles and human beings. Not only telegraphic and telephone messages but also computer data and multimedia content such as images can be transmitted through mobile communication.
In recent years, reduction in size and cost of mobile terminals has rapidly advanced owing to the improvement of manufacturing technology and the like. Expansion of information communication services and so on causes mobile terminals to be personalized, like mobile phones. Additionally, deregulation of communication and reduction of communication fee causes increased growth in user population.
Basically, in mobile communication, a mobile station, such as an in-vehicle phone and a mobile phone, detects the nearest base station to communicate radio waves between the mobile station and the base station. A range over which the radio waves sent from one base station can propagate is referred to as a “cell”. The cell is ordinarily a circle having a predetermined radius around an antenna mounted at the base station. Arrangement of cells with no space therebetween constitutes a communication service area.
FIG. 15 schematically illustrates the cell structure in a mobile radio communication system, typified by a cellular system, in which the service area is two-dimensionally expanded with a plurality of base stations. Providing base stations (not shown) at predetermined intervals and continuously (non-sparsely) arranging a plurality of cells offered by the respective base stations, as shown in FIG. 15, allow a mobile station to communicate with the base stations from anywhere, thus constituting a wide service area.
The advantages of the cells that are used in the mobile communication system include the following ones; that is, the propagation of the radio waves that are sent from a base station only within the corresponding cell causes the same frequency to be repeatedly used in other cells for efficiently utilizing limited frequency resources, and the division into the cells causes the radio-wave output for communication to be decreased for reducing in size of a mobile body that is usually installed as a battery-driven mobile device and for saving electric power. Recently, the reduction in size of the cells is increasingly advanced owing to the increase of the number of mobile phone users and so on.
A plurality of mobile terminals exists in one cell and they simultaneously communicate with one base station. In other words, from the point of the base station, it is necessary to detect which signal is transmitted from each user by multiple-access, that is, by multiplexing radio signals (multi-user detection).
Multiple-access technologies in radio communication include time division multiple access (TDMA) and frequency division multiple access (FDMA).
The TDMA is a communication system in which the communication channel is divided in advance into time slots on a time basis and different time slots are allocated to the respective mobile terminals, which simultaneously communicate with each other. The TDMA is assumed to be a digital system.
The FDMA is a communication system in which different frequencies are allocated to the respective mobile terminals, which simultaneously communicate with each other, (that is, different frequencies are allocated for every communication channel), to establish communication. Namely, multiple channels that are used for communication are arranged on a frequency basis and idle channels are appropriately allocated to the respective mobile terminals for use. The FDMA is accommodated to either analog or digital communication system.
For example, in the same cell, a base station can simultaneously connect to a plurality of mobile stations in its local cell by the TDMA. In such a case, one TDMA frame is divided into a plurality of time domains, each being allocated to an uplink or a downlink of each mobile station. A method, for example, channel allocation is employed for the allocation of the time domains.
Between cells, switching a frequency channel that is used to another frequency channel for communication by the FDMA allows the problems of interference waves from outside the local cell around the cell boundary to be eliminated.
Within one cell, the base station transmits (broadcasts) a beacon (identification signal) or other control information to perform synchronization within the local cell, identify the cell, specify the frequency channel to be used, and so on.
How to enhance the communication capability with fewer resources poses a big task in a radio communication environment in which the mobile communication has become quickly widespread and multiple mobile stations exist in one cell. In such a radio communication environment, the multi-user detection for detecting a plurality of users in a cell is highly important. With a known radio communication system, there is no benefit given by multiplexing signals for someone other than a desired person for transmission. This is because the interference is simply increased to lower the communication quality. In other words, there is no known radio communication system on the assumption of the multi-user detection and of the synthesis of user data and the control information to transmit the synthesized data.
Meanwhile, in the service area in which small cells are arranged with no space therebetween as shown in FIG. 15, the base station to be connected must be shifted in connection with a mobile station that moves from one cell to another cell. Such a shift of the base station to be connected at a mobile station is referred to as “handover” or “handoff”. The opportunity for the handoff is increased as the cells are downsized.
The mobile station must determine the base station to be linked to next, that is, must perform cell detection, for every handoff. For example, the move of the mobile station toward the cell boundary causes the communication quality of desired waves from the local base station to be lowered and permits the reception of interference signals from outside the local cell, so that it is necessary for detecting another base station having higher communication quality. The cell detection is performed also on startup of the device.
In a known cellular system, the switching of the control information between base stations through a control-switching station starts the handoff procedure. FIG. 16 is a diagram showing an example of a multi-cell environment in a cellular communication system. Referring to FIG. 16, a mobile station 201 moves toward a cell 13 while communicating with a base station 102 in a cell 12. The function for switching the target of the communication signal for a subscriber station from the transceiver in the original base station 102 to the transceiver in a next transceiver 103, when the mobile station 201 is to cross the cell boundary, is referred to as “handoff”.
The handoff techniques in the cellular communication system are mainly divided into two categories; “soft handoff” and “hard handoff”. With either handoff technique, the communication must be established between the base stations for informing that the mobile station is a target for the handoff when the mobile station switches the base station to be connected in the cell boundary. Such communication between base stations is established through a backbone. Namely, an infrastructure, such as a mobile communication control center, for controlling the base stations is necessary, as shown in FIG. 17.
The hard handoff ordinarily occurs in the vicinity of the cell boundary. In this case, the base station continuously measures the power of the received signal from the mobile station that is communicated with and determines whether the power of the received signal from the mobile station in the vicinity of the cell boundary is lower than a rated value. The base station in the candidate cell to be switched to and the base station in the original cell must communicate the information concerning the handoff with each other through the backbone in order to quickly perform the handoff without suspending the call in progress.
The soft handoff is continuously performed within a range apart from the base station in the original cell and the base station in the cell to be switched to by a predetermined distance. In the soft handoff, when the mobile station moves in the vicinity of the boundary between cells, it is connected to both base stations in the respective cells. It is determined whether the switching is performed based on the reception of a pilot signal from the mobile station. A telephone switching center, that is, the control-switching station that exists on the backbone of the corresponding network determines the point where the switching from the base station in the original cell is performed.
With either handoff technique, it is essential to establish some kind of connection between the base stations in order to realize the cellular handoff in a multi-cell environment. Hence, the number of control-switching stations is increased in accordance with the increase in the number of the base stations, thus causing a problem in that the cellular network is complicated. Since the shift to higher frequency band has been advanced in accordance with the request for high capacity and the request for high communication speed in recent years, it is anticipated, based on the propagation characteristic or the like, that the size of the cells is reduced, that is, the number of the base stations is increased. The reduction in traffic of the backbone that connects the base stations is an important concern.
When the number of users connected to a base station increases (that is, the cell is overloaded) and the number of users connected to another base station next the above base station is small, a control method is proposed for forcedly performing the handoff of the mobile station that is connected to one base station while being located in the vicinity of the boundary between the base stations, to the other base station. Also in such a case, the resource management information at each base station must be communicated through the control-switching station by using the resources in the backbone, thus increasing the traffic in the backbone.
In order to expand the calling areas with no space therebetween by using a known handoff technique, the constructor of the base station (access point) must allocate the frequency according to planning such that the base station in a neighboring cell does not use the same frequency for eliminating the interference. The infrastructure is designed so as to reduce the load of the mobile station by providing the base station that is fixedly installed with various control functions. Under such design of a radio span, the mobile terminal is obliged to rely on the base station as for the control, so that the mobile-station-driven control operation cannot be expected in a radio layer.